Frequently, sulfur oxide gases, such as sulfur dioxide (SO.sub.2) and sulfur trioxide (SO.sub.3), must be removed from a gas stream. The costs associated with a sulfur oxide removal apparatus, such as a water scrubber or bubbler, increase as the amount of gas treated increases. At low sulfur oxide levels, removal of the sulfur oxide gases is difficult.
Increasing the sulfur oxide gas concentration (or producing a more concentrated sulfur oxide gas stream) prior to sulfur oxide gas removal should reduce these costs and difficulties. Furthermore, sufficiently increasing the sulfur oxide gas concentration may permit more efficient processing of sulfur oxide gases to form marketable byproducts such as sulfuric acid, gypsum, or sulfur.
One method for removing sulfur oxide gases from a gas stream involves the sorption of sulfur oxide gases on a zeolite, which is a molecular sieve. Sorption is a process which may involve both absorption and adsorption. The zeolite should have a high sorptive capacity for sulfur oxide gases and should be stable in an acid environment because of the hydration of some sulfur trioxide to sulfuric acid. A zeolite with a high silica content is resistant to acid degradation. Mordenite, (ideally NaAlSi.sub.5 O.sub.12.3H.sub.2 O), is one of a variety of zeolites which has been recognized as having desirable properties for the sorption of sulfur oxide gases. Mordenite has several varieties, including sodium mordenite.
Tamboli and Sand (1970) and Blodgett (1972) established adsorption isotherms for different zeolites. They found that sodium mordenite sorbed about 20 weight percent sulfur dioxide at 25.degree. C. and at a sulfur dioxide pressure of 2 mm Hg (2600 ppm). Mordenite was found to be acid resistant and therefore recyclable. Synthetic zeolite 4.ANG. was found to have a higher sulfur oxide capacity than other zeolites, but it was attacked by acid.
Hawkins (1988) reported that the gas-sorption data for mordenite from Alaska follows a Freundlich adsorption isotherm The constants given indicate that at 25.degree. C. the Alaskan mordenite would sorb about 1.7 weight percent sulfur dioxide at a sulfur dioxide pressure of 2 mm of Hg. and about 9.5 weight percent sulfur dioxide at a sulfur dioxide pressure of 700 mm of Hg.
In these systems, the sorbent is heated to desorb the sulfur oxide gases. This requires enough energy to heat the sorbent (e.g. mordenite) to the temperature at which the previously-sorbed sulfur oxide gases are volatilized. Because it is difficult to instantaneously heat the zeolite bed sufficiently to cause rapid and complete desorption of the sulfur oxide gases, obtaining a gas stream with a high sulfur oxide gas concentration is difficult. Moreover, extended heating of the bed with sorbed sulfur oxides is thought to reduce bed life.